This invention relates to an optical signal reproducing system for video discs, and more particularly to a system of this kind having tilt immune means for correcting any lateral offset of the scanning beam to make it precisely follow the recording track.
In recent years, an electric signal reproducing system has been developed utilizing an optical beam as the reproducing media. A conventional optical signal reproducing system of this type is shown in FIG. 1, wherein a light source such as an He-Ne laser generates a beam 2 which is converted into a linearly polarized beam 4 by a collimating lens 3 and is incident upon a quarter-wave plate 8 through a polarized beam splitter 5. After passing through the quarter wave plate the laser light is collected into a light spot 10 on a disc 11 by a collimating lens 9. The disc 11 stores electrical signals such as TV signals, PCM audio signals and the like on its recording track composed of a series of pits 13, and is positioned near the focus of the collimating lens 9. When the light is incident upon the disc 11, the reproducing beam bearing recorded signals is reflected from the disc and reversed through its optical path to again pass through the collimating lens 9 and be polarized at the quarter-wave plate 8 by 90 degrees from the original laser light beam 6. After reaching the polarized beam splitter 5, the reflected light beam 7 advances on its optical path which is turned through an angle of 90 degrees by the beam splitter. This reflected light beam 7 is divided and converted into two electrical signals by a dual optical detector 16 having two detectors 16a and 16b, whose outputs are fed to both a summing amplifier 19 and a differential amplifier 17.
In this system, as the disc 11 is rotated by an electric motor 14, the light spot 10 traces the recording track and the reflected light is attenuated when the spot impinges on a pit 13 since the depth of the pit is chosen to be other than a multiple of a quarter-wavelength integer. On the other hand, when the spot 10 impinges on the area between the pits, or on what is termed a land, the intensity of the reflected light is greater in comparison with that for the pit. Thus, electrical signals stored in the recording track 12 in the form of pits are reproduced as the output 20 of the summing amplifier 19. If the light spot 10 is focused off of the recording track 12, however, the output on line 18 produced by the differential amplifier 17 causes a collimating lens transducer 15 to move in a direction A perpendicular to the optical axis and transverse to the recording track 12. The corrective movement of the collimating lens is substantially proportional to the amplitude of the differential output on line 18, such that the accurate or properly centered tracking of the light spot 10 will be restored.
FIG. 2 shows a reflected light beam 7 positioned on the dual optical detector 16, in which a division line 30 of the detector concides with the central line or diameter of the reflected beam, and the projection of the recording track 12 incident upon the detector is parallel with the line 30. With such an arrangement, the distribution of the light beam on the detector 16 is as illustrated in FIG. 3(a) if the optical axis is perpendicular to the disc; the light spot 10 and recording track 12 are overlapped one upon the other, and with the light spot located on a pit 13 the light beam is symmetrically distributed on both sides of the division line.
If the light spot is similarly aligned with the center line of the recording track but incident on a land, the light beam is symmetrically distributed on both sides of the light detector as shown in FIG. 3(b). If the light spot is laterally shifted from the recording track and incident on a pit, however, the light beam is distributed as shown in FIG. 3(c) whereby the detectors 16a and 16b receive different luminous intensities. If the light spot is laterally shifted from the recording track and incident on a land, the light beam distribution on both detectors is again symmetrical as shown in FIG. 3(b), with the identical light intensity incident on each detector half. Thus, only when the light spot is incident on a pit can the distribution of the reflected beam on both detectors be asymmetrical as a function of the lateral offset between the recording track and the light spot. The differential output on line 18 between the dual light detectors 16a and 16b is positive or negative according to which side the light spot is offset relative to a pit, and with the light spot on a land a zero differential output will result even with the track being offset. After low-pass filtering the differential output will vary with respect to the tracking offset as shown in FIG. 4, and if such output is impressed on the collimating lens transducer 15 with proper polarity, any offset of the light spot 10 out of the recording track 12 will be corrected. In other words, a negative feedback is applied such that the light spot will be continuously centered on the recording track.
An optical signal reproducing system as described above has a serious drawback when the optical axes of the disc 11 and collimating lens 9 are not perpendicular to each other (hereinafter referred to as the disc being tilted or listing).
FIG. 5 shows the distribution of light intensity on the dual detectors with the disc listing by 0.5.degree.-1.5.degree.. FIGS. 5(a) and (c) show the respective distributions in the cases of track listing but no lateral offset and track listing with lateral offset, both with the light spot positioned on a pit. FIG. 5(b) shows the distribution with the light spot positioned on a land, each of the distributions having about the same shapes as in their FIG. 3 counterparts. With reference to FIG. 5, if the light spot is positioned on a pit the differential output on line 18 due to a tracking offset will be much greater than the output due to disc listing, but if the light spot is positioned on a land a relatively large differential output is generated purely as a function of disc listing (compare the areas under the distribution curve on both sides of the ordinate in FIG. 5(b)). In the conventional system, the tracking offset correction signal on line 18 will be the mean value between the differential signal produced with the spot on a pit and the one produced with the spot on a land, whereby if the disc is not tilted no erroneous or over-correction will result. If the disc is tilted, however, the differential output generated on a land where the intensity of the reflected light is relatively large becomes an overriding factor, and since this output component is solely due to disc listing an erroneous centering correction will result even when there is no tracking offset.